Marcus Webster's added CURLFORM_CONTENTHEADER docs

docs/curl_formadd.3

@@ -58,6 +58,13 @@ are allowed. The effect of this parameter is the same as giving multiple
 \fBCURLFORM_FILE\fP options possibly with \fBCURLFORM_CONTENTTYPE\fP after or
 before each \fBCURLFORM_FILE\fP option.
 
+Should you need to specify extra headers for the form POST section, use
+\fBCURLFORM_CONTENTHEADER\fP. This takes a curl_slist prepared in the usual way
+using \fBcurl_slist_append\fP and appends the list of headers to those Curl
+automatically generates for \fBCURLFORM_CONTENTTYPE\fP and the content 
+disposition. The list must exist while the POST occurs, if you free it before
+the post completes you may experience problems.
+
 The last argument in such an array must always be \fBCURLFORM_END\fP.
 
 The pointers \fI*firstitem\fP and \fI*lastitem\fP should both be pointing to